FIG. 1 is a conventional circuit for an optical sensor, which uses an active pixel sensor (APS) 10 as an optical detector, and a correlated double sampling (CDS) circuit 12 to eliminate fixed pattern noise (FPN) caused by mismatch in the manufacturing processes of the active pixel sensor 10. The active pixel sensor 10 includes a photodiode Dphoto, a reset switch Q1, a source follower Q2 and a position select switch Q3. Before an optical sensing process begins, a signal ROW_RST turns on the reset switch Q1 to reset the voltage Va at the cathode of the photodiode Dphoto, and then the reset switch Q1 is turned off so as for the optical detection of the photodiode Dphoto, by which the photodiode Dphoto will generate a photoelectric current responsive to the optical intensity and thereby discharges the cathode Va thereof, and as a result the voltage Va is decreased by ΔVa, which is proportional to the optical intensity imparted on the photodiode Dphoto. The source follower Q2 shifts the level of the voltage Va to be a voltage Vo, which will be sent out when the position select switch Q3 is turned on by a signal ROW_SEL. In the process of sampling the output voltage Vo of the active pixel sensor 10 by the CDS circuit 12, a signal SHS turns on a switch for a period of time so that the voltage Vo generated by the optical detection of the photodiode Dphoto is stored into a capacitor CS to generate a sampled voltage VS, then a signal SHR turns on another switch for a period of time so that the voltage Vo generated when the photodiode Dphoto is reset is stored into a capacitor CR to generate a sampled voltage VR, and a signal COL_SEL controls yet another switches to send out the sampled voltages VR and VS. As the sampled voltage VR results from FPN and the sampled voltage VS includes both FPN and the voltage generated by the optical detection of the photodiode Dphoto, the difference thereof, i.e. VS−VR, is the actual optical sensed value excluding FPN. Thus, image quality can be effectively enhanced.
FIG. 2 is a circuit diagram of a conventional active image sensor, which includes an array 14 of active pixel sensors 10, an array 16 of sampling circuits 12 and a readout circuit 18. For example, the circuits of the active pixel sensor 10 and the sampling circuit 12 are the same as that shown in FIG. 1. The number of the sampling circuits 12 in the array 16 is equal to the number of the active pixel sensors 10 of a row in the array 14. For example, the array 14 is composed of 256 active pixel sensors 10 arranged as a 16×16 array, and the array 16 is composed of 16 sampling circuits 12 arranged as a 16×1 array. FIG. 3 is a timing diagram of the active image sensor shown in FIG. 2. In reading out the optical sensed values of a frame from the array 14, signals ROW_SEL[1]-[16] are used to select the rows in the array 14 one by one, so as for the array 16 to sample the optical sensed values of the selected row. Each time the array 16 samples the optical sensed values of a row, and the sampling time of each row is T1. After the array 16 samples the optical sensed values of a row, the readout circuit 18 uses signals COL_SEL[1]-[16] to select the columns one by one to read out the sampled signals of all the pixels in the array 16, and the readout time of each row is T2. Then, the readout circuit 18 will convert the sampled signals into an 8-bit digital signal DO[7:0] and send out the digital signal DO[7:0] together with a synchronous clock ADEND. As the time required to sample and read out the pixels in each row is the sum T1+T2 and the array 14 has 16 rows, the time required to read out the entire array 14, i.e. the readout time of a frame, is (T1+T2)×16.